1. Technical Field
The present invention relates to a composition containing uniformly dispersed, finely sized, liquid-state-in-situ-formed ceramic particles in metal and metal alloys, and to products containing the uniformly dispersed, finely sized ceramic particles formed in metal and metal alloys by the liquid-state in-situ process of the present invention. In one aspect, the present invention relates to a composition containing uniformly dispersed, finely sized, liquid-state-in-situ-formed titanium carbide particles in aluminum and aluminum alloys, and to products containing the uniformly dispersed, finely sized titanium carbide particles formed in aluminum and aluminum alloys by the liquid-state in-situ process of the present invention.
2. Background
The aluminum and aerospace industries have long sought a method to control recrystallization of aluminum alloys during deformation operations to permit the design of aluminum airframes with improved structural properties.
The metals industry today conventionally uses dispersoids, i.e., fine particles dispersed in the metal alloy, to control recrystallization and to increase dispersion strengthening at elevated temperatures. Such dispersoids of fine particles dispersed in the metal alloy usually are formed by solid state precipitation.
Recent developments in this area suggest that to improve formability and high temperature strength of aluminum alloys, it is necessary to increase the number densities and to reduce the size of the fine particle size dispersoids.
Conventional processes have the ability to form only a limited level of particle number density, because the number density of the dispersoid is determined by the initial dispersoid forming elements content as limited by its equilibrium solubility in liquid metal during solidification. For example, at the typical solidification rate in the range of about 0.05xc2x0 C./sec to 20xc2x0 C./sec, the maximum solubility of zirconium in aluminum is about 0.12 wt. percent, which is considered to be entirely too low for processing at higher temperatures and to form preferred structural properties. Accordingly, a process having the ability to form higher level number densities of stable particle is desired.
In the aluminum industry, dispersoid-forming elements such as Mn, Zr, Cr, V, Ti, Sc, and Hf are added to aluminum to increase the resistance to the recrystallization and increase the recrystallization temperature and to control the grain structure in cast and wrought products. Conventionally, different methods have been employed to add these dispersoid-forming types of alloying elements to molten aluminum metal. Historically, master alloys containing the desired elements have been added directly to the melt in the forms of a solid lump or bar.
The alloying elements in the master alloys normally are present in the form of coarse intermetallics, and these intermetallics require superheat and a long period of holding time to be dissolved in the melt. The heavy intermetallics also tend to settle to the bottom of the holding furnace by force of gravity. For this reason, the master alloys generally are added at a process location up-stream from the molten metal filters so that any coarse intermetallics which do not dissolve in the furnace can be removed prior to casting. If these coarse intermetallics are not filtered out of the molten metal, they adversely affect the mechanical properties of the solidified material. Removal of intentional alloying additions is inefficient and expensive. Perhaps more importantly, however, coarse intermetallic particles do not provide the preferred metallurgical benefits provided by the finer dispersoid particles.
Silicon carbide and alumina are the most commonly used reinforcement particulates. Certain emerging technologies are capable of producing fine particulates of different types with somewhat improved interfacial characteristics. Among the several ways of producing these materials, the technologies where the particles are introduced or formed in the molten aluminum prior to its solidification are attractive, primarily because of the potential for commercially economic processes on a large scale.
A variety of processing routes classified generally as in-situ ceramic phase formation processes in metal have been developed recently. According to the state of the reactants in the process, such a ceramic phase formation process in metal generally is classified into one of several categories:
(1) liquid metalxe2x80x94gas reaction,
(2) liquid metalxe2x80x94liquid metal reaction, or
(3) liquidxe2x80x94solid reaction.
In the case of carbon particles or carbon blocks in the context of liquid metalxe2x80x94liquid metal reactions or liquidxe2x80x94solid reactions, it is known that such carbon particles or carbon black are difficult to introduce directly into a melt in metal because of non-wetting of the carbon by the molten metal or alloy.
Recent developments in liquid metalxe2x80x94gas reaction processes have produced fine TiC particulates in a molten aluminum alloy. In this approach, a carbonaceous gas is introduced into an aluminum melt containing titanium to form TiC particulates, and the carbide volume fraction is determined by the initial titanium content. When the melt containing the carbides is cast and subsequently extruded for microstructure and property evaluation, the as-cast microstructure of the in-situ processed composites reveals a relatively uniform distribution of TiC particles with an average size of a few microns. No preferential particle segregation is observed in the dendritic cell boundaries generally.
U.S. Pat. No. 4,808,372, issued to Koczak et al., discloses an in-situ process for producing a composite containing refractory material. A molten composition, comprising a matrix liquid, and at least one refractory carbide-forming component are provided, and a gas is introduced into the molten composition. Methane is bubbled through a molten composition of powdered aluminum and powdered tantalum to produce a carbide having an average particle size in the fine mode of about 3 to 7 xcexcm and in the coarse mode of about 35 xcexcm.
Although conventional ceramic phase formation processes in metal offer some possibilities for the production of a wide range of reinforcement particle types and improved compatibility between the reinforcement and the matrix, the in-situ formed ceramic particles in metal are too large, e.g., on the order of several microns, and tend to form clusters. In-situ formed ceramic particles having these sizes, i.e., of several microns, are candidates for use as reinforcement in a composite, but are not suitable for use as dispersoids for recrystallation control, for dispersion strengthening, or for use as a component for structure refinement.
Accordingly, a novel ceramic dispersoid in metal product and process for making such a novel ceramic dispersoid in metal product are needed for providing uniformly dispersed, finely sized ceramic phase particles dispersed in-situ in a metal matrix.
U.S. Pat. No. 4,842,821 and U.S. Pat. No. 4,748,001, issued to Banerji et al., disclose a method for producing a metal melt containing dispersed particles of titanium carbide. Carbon particles are reacted with titanium in the metal to obtain titanium carbide. Banerji discloses that salts preferably are entirely absent from the melt (U.S. Pat. No. 4,842,821 Col. 3, lines 26-28, and U.S. Pat. No. 4,748,001 Col. 3, lines 40-42). The Banerji reference discloses a salt containing titanium (K2TiF6) as opposed to a component mixture of a salt together with carbon particles.
U.S. Pat. No. 5,405,427, issued to Eckert, discloses a flux composition for purifying molten aluminum to remove or capture inclusions in the melt and carry such inclusions to the surface (Col. 4, line 13 et seq.). The flux composition contains sodium chloride, potassium chloride, and a minor amount of magnesium chloride and carbon particles.
U.S. Pat. No. 5,401,338, issued to Lin, discloses a process for making metal matrix composites wherein fine particles (0.05 xcexcm) of alumina, silicon nitride, silicon carbide, titanium carbide, zirconium oxide, boron carbide, or tantalum carbide are added into a metal alloy matrix (Col. 2, lines 64-68).
U.S. Pat. No. 5,041,263, issued to Sigworth, discloses a process for providing a grain refiner for an aluminum master alloy that contains carbon or other third elements and acts as an effective refiner in solution in the matrix, rather than being present as massive hard particles.
U.S. Pat. No. 4,917,964, issued to Moshier et al., discloses producing titanium carbide by induction heating a powder of titanium, carbon, and aluminum, i.e., in the form of a compact, to produce a concentrate of 60 wt. % titanium carbide in the form of a solid which is crushed. (Moshier Example 7, Col. 21-22.) The other Moshier actual examples, i.e., Examples -6 and 8, are similar, but use boron and not carbon. The Moshier additives are added as a solid phase powder, not as a liquid phase.
U.S. Pat. No. 4,915,9086 issued to Nagle et al. discloses a Direct Addition Process which adds a powder of titanium, e.g., a compact, to aluminum as molten aluminum and powder aluminum. See Nagle Examples 1-5, col. 16-17. The Nagle additives are added as a separate phase, i.e., in a solid phase powder different from the molten phase of the matrix. The Nagle process is highly exothermic and difficult to control. Nagle does not teach the volume of matrix aluminum metal or the volume of titanium carbide ceramic phase particles.
U.S. Pat. No. 4,885,130 issued to Claar et al. discloses filtering a parent metal into a boron donor material. Claar does not teach a uniform cluster-free distribution of no more than two particles attached to one another at a magnification of 500xc3x97. Claar does not teach particle sizes of the ceramic phase particles in the final metal matrix. Claar mentions particle size in only one place, i.e., col. 10, line 69, which refers to a particle size of the Claar preform. When Claar refers to particle size, they are referring to the preform, not the final product. Rather, Claar is referring to the particle size of the precursor of the product to be formed. The Claar Examples nowhere mention particle size. Such a ceramic preform formed from a particle compact as used in Claar is very porous. It is very porous because it is used to filter molten metal into it to make the composite. During filtration, Claar needs to have a reaction between the molten metal and the particulate ceramic preform to form the composite.
When Claar refers to particle size, they are referring to the preform, not the final product. Rather, Claar is referring to the particle size of the precursor of the product to be formed. The Claar Examples nowhere mention particle size. The difference can be seen further at col. 11, line 32, in reference to a volume fraction which reacts. Claar is referring to a volume fraction which reacts, not the volume fraction of the final product i.e., the final ceramic dispersoid in metal product.
Uniformly high number densities of finely sized dispersoids increase the recrystallization temperature, inhibit grain growth in hot working, and improve elevated temperature strength. Further, fine particles of dispersoids are effective nuclei for grain refining.
It is against this need in the background technology that the present invention was made.
Accordingly, it is an object of this invention to provide aluminum alloys having high number densities of dispersoids.
Accordingly, it is an object of the present invention to provide a method for increasing the number densities of dispersoids in the liquid state and which then remain stable and dispersed in the solid state in metal alloys.
It is an object of the present invention to produce finely sized ceramic phase particles.
It is a further object of the present invention to produce uniformity in the dispersion of finely sized ceramic phase particles in metal and in alloys.
It is yet another object of the present invention to produce uniformly distributed, finely sized ceramic phase particles dispersed in-situ in a metal matrix.
It is another object of the present invention to produce uniformly distributed, finely sized ceramic phase particles dispersed in-situ in a metal alloy in a process providing reaction times shorter than conventional approaches.
It is another object of the present invention to produce uniformly distributed, finely sized ceramic phase particles dispersed in-situ in a metal alloy for recrystallization control, dispersion strengthening, or grain refining.
These and other objects of the present invention will become apparent from the detailed description which follows.
The present invention provides a novel method for producing a ceramic phase particle dispersoid in metal and a novel product composed thereof, including finely sized carbide phase particles formed in situ in a molten metal by a salt-based liquid state reaction with Ti, B, Si, Sc, V, Hf, Nb, Ta, Zr, Mo, or Al (when the molten metal matrix is not aluminum), and a halide salt containing fine carbon particles to form a uniform distribution of finely sized ceramic phase particles formed and dispersed in-situ in the metal matrix. The ceramic dispersoid in metal product of the present invention includes at least about 50 volume percent of a matrix metal of aluminum metal or aluminum alloy and up to about 50 volume percent of a uniform distribution of finely sized carbide ceramic phase particles formed and dispersed in-situ in the aluminum metal matrix, wherein the finely sized ceramic phase particles have an average particle diameter of less than about 2.5 microns, and wherein the uniform distribution consists of a substantially cluster-free distribution of no more than two particles attached to one another at a magnification of 500xc3x97.